What is an oligomer?

An oligomeric protein consists of few repeating units, as opposed to a polymer, which has many units, or monomer, which has only one unit. Oligo– is derived from the Ancient Greek word meaning “few” and mer comes from the Ancient Greek word for “part.”

Alpha synuclein is a protein implicated in Parkinson’s Disease (PD), a common neurodegenerative disorder. While alpha synuclein is normally found in the presynaptic terminals of neurons, in certain diseases, known as synucleinopathies, it aggregates to form larger fibrils and aggregates called Lewy bodies. During this aggregation process, it forms intermediate species called oligomers. These oligomers are of interest because they, rather than Lewy bodies, are increasingly thought to be the toxic species that cause neuron death in synucleinopathic diseases.^1,2

How Dopamine Induces Alpha Synuclein Oligomerization

Dopamine is a neurotransmitter that can inhibit the aggregation of alpha synuclein into fibrils, stabilizing it in its oligomeric form. Dopamine oxidation is involved in this process.

Dopamine Oxidation Mechanism

The first step in dopamine oxidation is the oxidation of the catechol ring to form dopamine-quinone. This species can then react to form dopaminochrome, which then can be converted into 5, 6-indoloquinone and ultimately neuromelanin.³

Interactions with Alpha Synuclein: Oligomer Stabilization

Dopamine and its derivatives are thought to inhibit alpha synuclein aggregation and kinetically stabilize oligomers through interactions in different regions:

C-terminus: Y125EMPS129

Computational modelling indicates that the aromatic ring of dopamine interacts with this 5-amino acid sequence of alpha synuclein in a non-specific, non-covalent, hydrophobic manner.⁴ However, there is also evidence that the oxidation of the methionine residue in this region, and not binding with dopamine, that inhibits fibrillization.⁵ In any case, this motif is required for oligomer stabilization^6-9 and the mutation of Y125EMPS129 to F125AAFA129 prevents this by preventing interaction with dopamine.^7,11

NAC Region: E83

Dopamine derivatives are thought to also have long-range electrostatic interactions with E83 in the NAC region.⁴ The mutation E83A impairs dopamine’s ability to inhibit fibril formation.⁴

Oligomerization Mechanism

Dopamine and its derivatives not only kinetically stabilize oligomers, but can also induce oligomerization of monomers. Dopamine can induce dimerization, with residues 43 through 60 in the N-terminal forming the dimer interface.¹⁰ Dopamine also induces trimerization, where monomers overlap but do not have associate end-to-end.¹¹ The oxidation of four methionine residues at the C and N-terminals prevents end-to-end associations and beta-sheet formation by cross-linking with dopamine-quinone and dopamine-melanin.¹¹ Dopaminochrome can induce reversible conformational changes that promote oligomer formation¹² and DOPAL, a toxic dopamine metabolite, is thought to promote monomer aggregation by cross-linking alpha synuclein lysine residues.¹³

Alpha Synuclein Oligomer Structure

There are many varieties of alpha synuclein oligomers, both found in living systems and synthesized in labs. Alpha synuclein oligomers derived from post-mortem brains can range from dimers (two units) to 150 units.^1,14

Evidence for Oligomer Toxicity

Alpha synuclein oligomers are increasingly recognized as a toxic species in synucleinopathies. In vitro experiments and animal models have demonstrated their toxicity, as cell death and degeneration occurs when there are oligomers present, even in the absence of large alpha synuclein aggregates.^15-18

However, there are also non-toxic varieties of oligomers. The most toxic oligomers are thought to be between 2 and 6 nm in diameter,¹⁴ whereas larger oligomers are thought to be non-toxic but capable of seeding.¹

Alpha Synuclein Oligomer Toxicity Mechanisms

Alpha synuclein oligomers can be toxic to cells through various mechanisms:

Interactions with Lipid Membranes and Pore Formation

Alpha synuclein oligomers have exposed hydrophobic surfaces¹⁹ and accessible N-terminal regions that enable them to interact with and destabilize lipid membranes.²⁰ Small, annular oligomers are likely to form pores in vesicle and plasma membranes.^21-25 Oligomer hydrophobicity is thought to be associated with toxicity since hydrophobic oligomers can bind more tightly to membranes and organelles.²⁶ This would account for the decreased toxicity in larger aggregates with fewer accessible hydrophobic patches.

Calcium Influx

Alpha synuclein oligomers can induce Ca²⁺ influx via N-type voltage-dependent Ca²⁺.²⁷ This process is thought to be mediated by the NAC region.²⁸ While alpha synuclein monomers can also affect calcium signalling, only oligomers induce cell death.²⁹

Mitochondrial Dysfunction

Alpha synuclein oligomers can cause mitochondrial dysfunction by inducing a slow and progressive increase in NADH.³⁰ This is associated with permeability transition pore (PTP) opening, increased production of reactive oxygen species, inhibition of complex I, mitochondrial depolarization, and reduced electron flow through the electron transport chain.³⁰

Endoplasmic Reticulum (ER) Stress

Alpha synuclein oligomers can form within the ER lumen and accumulate within the ER, increasing neuronal sensitivity to ER stress.³¹ This ER stress contributes to neurodegeneration.

Glutamatergic Receptors

Globular oligomers can alter the function of glutamatergic receptors, which leads to disrupted neuronal function.^31,32

SNARE Complex Inhibition

Large intracellular dopamine-stabilized alpha synuclein oligomers can inhibit the formation of the SNARE complex, which reduces the release of neurotransmitters.³³

Alpha Synuclein Oligomer Seeding

Different varieties of oligomers show different seeding capacities. Oligomers with higher seeding capacities are associated with lower toxicities.³⁴ Intracellular alpha synuclein oligomers can be released via exosomes, where they are more likely to be taken up by neighbouring cells than free extracellular alpha synuclein oligomers.³⁵

Dopamine-Stabilized Alpha Synuclein Oligomers

Dopamine-induced oligomers have been shown to auto-aggregate³⁶and can cross-seed amyloid-beta aggregates.³⁷ Dopamine-induced oligomers have also induced neurodegeneration in worm and mouse in vivo models.³⁸ However, it is worth noting that different experimental conditions produce different types of oligomers, and the toxicity is dependent on oligomer synthesis method and structure.

StressMarq manufactures commercially available dopamine-stabilized oligomers for alpha synuclein research. Oligomers are of interest to researchers because they show potential as a biomarker, since they are present in plasma and CSF, as well as a treatment target since they are a cause of neurodegeneration.³¹

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